1. Field of the Invention
The present invention relates to an air-fuel ratio control device for a vehicle engine which is operated mainly on a lean air-fuel mixture.
2. Description of the Related Art
Engines which are operated on a lean air-fuel mixture having an air-fuel ratio higher than a stoichiometric ratio in the main operating range of the engines are known as lean burn engines. Usually, the lean burn engines are operated on a lean air-fuel mixture, and when acceleration or high load operations are required, the air-fuel ratio of the mixture on which the engine is operated is switched to the stoichiometric ratio or lower (rich) ratio, so that a high engine performance can be obtained without a worsening of the exhaust emissions and fuel efficiency.
When an engine is not sufficiently warmed up, it is difficult to obtain a stable combustion in the cylinders on a lean air-fuel mixture. Therefore, usually a lean-burn engine is operated on a stoichiometric air-fuel mixture during warming up, and the air-fuel ratio is switched to the lean condition after the engine is fully warmed up. The timing at which the air-fuel ratio is switched to the lean mixture ratio, i.e., the completion of the engine warm up, is usually determined by detecting the temperature of the engine coolant, such as cooling water.
For example, Japanese Unexamined Patent Publication No. 58-48727 discloses an engine operated on a stoichiometric air-fuel ratio mixture when the temperature of the engine cooling water is lower than the predetermined value, and when the temperature of the cooling water reaches the predetermined value, the air-fuel ratio is switched to lean ratio.
Nevertheless, the actual factor which influences the condition of the combustion in the cylinders is the wall temperature of the combustion chamber, not the cooling water temperature, and therefore, if the air-fuel ratio of the mixture is determined by the cooling water temperature only, in some cases a stable combustion cannot be obtained. For example, if the engine is stopped after being fully warmed up, and then restarted within a relatively short time, sometimes the combustion becomes unstable and misfires occur. This is caused by the difference in the cooling speeds of the cooling water and the wall of the combustion chamber. Namely, due to a high specific heat, the cooling speed of the cooling water is low but the cooling speed of the wall of the combustion chamber is relatively high. Therefore, when the engine is stopped while in a fully warmed up condition, the temperature of the cooling water drops very slowly; in practice, the temperature of the cooling water does not change for several minutes. Therefore, in the prior art, if the engine is re-started after a short stop of several minutes, the air fuel ratio is made lean immediately after the engine start up, since the cooling water temperature is still higher than the predetermined value. Nevertheless, the wall temperature of the combustion chamber drops more rapidly, and even within short stops of several minutes, the wall temperature of the combustion chamber becomes too low for a stable combustion of the lean air-fuel mixture. Consequently, in the prior art, when the engine is re-started after a short stop, an unstable combustion or misfire sometimes occurs due to an inappropriate switching to the lean mixture.
To solve this problem, the switching of air-fuel ratio of air-fuel mixture fed to engine must be controlled in accordance with the wall temperature of the combustion chamber, but a reliable measurement of the wall temperature of the combustion chamber with conventional devices is very difficult, and thus a control in accordance with the wall temperature is not considered practical.